The midbrain periaqueductal gray (PAG) has been implicated in several important functions including pain modulation, defensive behavior, vocalization and reproductive behavior. Despite the fact that a great deal of information is available regarding the anatomical organization of this region, very little data exists concerning the relationship of neuropeptides and neurotransmitters to the anatomical circuitry of this important midbrain area. The long term goal of the proposed research is to elucidate the chemical organization of the PAG which underlies its pivotal role in analgesia through the use of anatomical, biochemical, pharmacological and behavioral techniques. We will reach this goal by completing the following specific objectives: 1) to analyze the possible relationship of the opioid peptides, endorphin, enkephalins and dynorphin, as well as the mu, delta nd kappa opiate receptors to both GABA-containing interneurons and PAG projection neurons using anatomical, biochemical, pharmacological and behavioral techniques; 2) to examine the possible relationship of GABA immunoreactive terminals and GABAA receptor immunoreactivity to PAG projection neurons using light and electron microscopic immunocytochemistry in combination with retrograde transport techniques and intracellular filling of retrogradely labeled cells; 3) to analyze the possible relationship of neurotensin immunoreactive terminals to opioid- and excitatory amino acid-containing neurons in the PAG by utilizing double-labeling immunocytochemical techniques; and 4) to analyze the release of neurochemicals from the nucleus raphe magnus in response to electrical stimulation of the PAG or morphine administration into the PAG by using in vivo microdialysis techniques. These studies will provide novel data that will not only further our understanding of the basic organization of this midbrain region but will also elucidate the neurochemicals involved at the level of the PAG and raphe magnus in PAG mediated antinociception. Such information is essential for developing improved therapeutic approaches to pain management.